1. Field of the Invention
The present invention relates to an improved method for preparing a high-purity monochloroacetaldehyde trimer.
2. Description of the Prior Art
Monochloroacetaldehyde which is useful as a raw material for the synthesis of medicines and agricultural chemicals is manufactured by a known method, i.e., by chlorinating acetaldehyde, para-acetaldehyde or vinyl acetate.
However, monochloroacetaldehyde is very unstable and is extremely liable to polymerize itself. For this reason, it is impossible to preserve monochloroacetaldehyde for a long period of time, and therefore, nowadays, this compound is kept in the state of an aqueous solution (Japanese Patent Laid-open No. 99336/1987).
However, monochloroacetaldehyde tends to bring about condensation polymerization even in the aqueous solution in order to develop a color. This fact impedes the preparation of high-quality monochloroacetaldehyde which is required in recent years.
An attempt has been made to form a trimer of monochloroacetaldehyde with the intention of preparing high-purity monochloroacetaldehyde which can be stored for a long term, by Natterer [Monatsh, 3, 461-464 (1882)]. In this attempt, the trimer of monochloroacetaldehyde is formed by shaking, together with concentrated sulfuric acid, high-purity monochloroacetaldehyde obtained by the reaction between monochloroacetal and oxalic anhydride, the volume of concentrated sulfuric acid being one half of that of monochloroacetaldehyde. When the thus formed trimer is heated, decomposition occurs to obtain pure monochloroacetaldehyde. However, in this report, any actual yield and any identification of a structure are not elucidated.
In the method suggested by Natterer, expensive monochloroacetal and oxalic anhydride are necessary to obtain pure monochloroacetaldehyde. In addition, according to experiments by inventors of the present application, the Natterer's method has the following disadvantages: In the direct reaction between monochloroacetaldehyde and concentrated sulfuric acid, operation is very difficult, and a great deal of black tar-like substance is formed. As a result, the yield of the desired monochloroacetaldehyde trimer is as low as about 20% disadvantageously.